INGRID

The INGRID detector [61] is composed of 16 identical modules, 14 of which form the vertical and horizontal axes of a cross, seven per axis, with the central module on each axis aligned with the beam axis. The remaining two modules are placed

Figure 2.8: ND280 detector complex. The INGRID detector is formed of the vertical stack of modules rising from the lowest floor in the foreground, spanning the lower and central floors, along with the horizontal span of modules on the central floor behind the vertical stack. The ND280 detector is located on the top floor, shown in the magnet open configuration. The magnetic coils and flux returns are opened to each side of the pit revealing the central basket portion of the detector [51].

Figure 2.9: Position of the INGRID detector modules. Seven modules are used in both the vertical and horizontal branches of the detector cross, these monitor the beam position. A further two modules are placed symmetrically off the cross axis to measure the axial symmetry of the beam [51].

symmetrically off-axis from the main cross to measure the beam’s axial symmetry, see Figure 2.9.

Each INGRID module is composed of 9 iron and 11 scintillator planes sandwiched between one another, with no iron plate being placed between the 10th and 11th scintillator planes. Each iron plane is square with a length of 124 cm on each side and a thickness of 6.5 cm, this gives a total target mass of 7.1 t of iron per module.

Each of the tracking scintillation planes is composed of two layers of plastic scintillator bars, with one plane orientated horizontally, the other vertically. 24 bars are used in each layer, and an individual scintillator bar has a cross sectional area of 1 cm by 5 cm. Each module is surrounded by a veto plane on each side to reject incoming particle tracks. The veto planes are constructed of 22 scintillator bars aligned along the beam axis, see Figure 2.10.

The scintillation bars were produced at Fermilab [62] and are made of extruded polystyrene doped with 1% of 2,5-diphenyloxazole (PPO) and with 0.03% of 1,4-di-(5-phenyl-2-oxazolyl)-benzene (POPOP). Each bar is surrounded by a

co-(a) INGRID internal layers. (b) Full INGRID module with with veto planes.

Figure 2.10: Exploded view of an INGRID module. 2.10a shows the iron planes in blue, separated by the grey scintillator planes. 2.10b shows a full module, with the surrounding veto planes shown in black [51].

extruded white reflective jacket made of polystyrene infused with TiO2. A 3 mm diameter hole runs the length of the scintillator bars for the insertion of a wave-length shifting (WLS) fibre which extracts the scintillation light produced in the bars. The WLS fibres are Kuraray Y11(200)M and have a diameter of 1.0 mm which couples to Multi-Pixel Photon Counters (MPPCs) for electronic read out.

MPPCs are described in detail in Section 2.2.2. Only one end of each WLS fibre is readout by an MPPC, with the uninstrumented end of each fibre and bar being painted with a reflective coating of ELJEN^r EJ-510 to increase total light yield.

For the beam power currently achieved, INGRID has measured the beam centre position on a month by month basis. This was found to be consistently stable within 28 cm, corresponding to beam direction being known to within 1 mrad as required for precision oscillation measurements. The neutrino event rate is also measured, this time on a day by day basis, and this is stable within statistical uncertainty which is typically 1.7% [61].

In addition to the standard INGRID modules, is an iron-free, higher granularity scintillator module. This is known as the proton module and is placed between the two central modules on each axis of the cross. The proton module is designed

its higher granularity tracking capabilities. The desire is to isolate quasi-elastic interactions in this module to compare with Monte Carlo simulations.

2.2.2 ND280

The ND280 lies slightly downstream of INGRID within the detector pit and is offset from the beam axis by 2.5°. The detector is housed in the refurbished magnetic coils and flux return yokes previously used by the UA1 [63] and NO-MAD [64] experiments (kindly donated by CERN). It is composed of two regions, the upstream π⁰-detector (PØD) region and downstream tracker region, both of which consist of several individual sub-detectors as seen in Figure 2.11.

Figure 2.11: Exploded view of the ND280 detector [51].

The ND280 has external dimensions of 7.6 m (l) by 5.6 m (w) by 6.1 m (h), as constrained by the size of the magnetic yoke. The internal basket is 6.5 m by

two magnet clams can be opened up allowing access to the internal detectors for installation and maintenance work, and then securely closed around the basket during experimental running.

The magnetic coils of the ND280 are held within the external magnetic yoke, with the Barrel Electromagnetic Calorimeters (BrECals) and π⁰-detector Elec-tromagnetic Calorimeters (PØDECals) being attached to the internal surface of the magnetic coils. The π⁰-detector (PØD) resides within the upstream end of the basket region and is surrounded by the PØDECals when the magnet is closed.

Behind the PØD are the tracking sub-detectors; these are the three Time Projec-tion Chambers (TPCs) which are separated by the two Fine Grained Detectors (FGDs). The Downstream Electromagnetic Calorimeter (DsECal) makes up the downstream face of the basket and is the final component of the tracking region, with the BrECals then surrounding the entire tracking region when the basket is closed.

Magnet

The ND280 magnet produces a 0.2 T dipole magnetic field; this allows high reso-lution measurements of the charge and momenta of charged particles within the detector’s tracker region. The field is produced by four water-cooled aluminium coils which sit, two per side, within the the return yoke. Each side of the return yoke is known as a clam and these are mirror-symmetric. Each yoke is segmented into eight C-shaped elements, each of which is made of low-carbon steel plates.

16 steel plate layers make up each yoke element, these are 48 mm thick and separated by 17 mm air gaps [65]. The total mass of the magnet and yoke is 850 t, dominating the total mass of the ND280 and therefore the site of the vast majority of neutrino interactions within the detector.

Figure 2.12: Image of an SMRD paddle prior to installation with the characteristic snaking WLS fibre running through it [65].

SMRD

The Side Muon Range Detector is a series of 440 scintillation paddles that are placed within some of the air gaps of the magnetic yoke. The SMRD has three primary functions; firstly it measures the momentum of muon tracks exiting the ND280 at high angles, secondly it acts as a veto for particles entering the de-tector from the outside and neutrino interactions within the magnetic yoke and surrounding rock, and finally it works as a trigger for incoming cosmic rays which are used in detector calibration and validation studies.

For the eight yoke elements that make up each side of the clam, the top and bottom portion of each yoke element have the three most internal layers (closest to the magnetic coil) instrumented with the scintillator paddles. Moving from upstream to downstream, the first five yoke elements have the three most internal layers of their sides instrumented, the sixth element has the four most internal layers instrumented, and the seventh and eighth layers have the six most internal layers instrumented. This bias in read out towards the downstream end of the detector gives better reconstruction for tracks in the forward going direction.

Each scintillator paddle is composed of the same combination of scintillating material, wavelength shifting fibre and read out as previously described for the INGRID detector, and is used by all the other scintillator sub-detectors within the ND280. The SMRD is unique though in the size of the scintillator paddles that it uses, with each being 7 mm deep, 875 mm long and with a width of 167 mm or 175 mm. This large width means that the WLS fibre snakes through the paddles as can be seen in Figure 2.12 and is read out from only one end.

marily tasked with detecting neutral-current neutrino interactions with associated π⁰ production (NCπ⁰). An accurate measurement of the NCπ⁰ cross-section on water is important, as this would constrain the uncertainty in the νe appearance studies at the T2K far detector, for which NCπ⁰ are a major background.

The PØD has an active region measuring 2103 mm wide by 2239 mm high by 2400 mm deep, and is built up of layers of scintillator bars, brass or lead sheets, and high density polyethylene (HDPE) water bags [66]. The target water bags can be filled and emptied as required, changing the detector mass between 15,800 kg and 12,900 kg, allowing a water-in/water-out NCπ⁰ rate difference to be calculated, leading to an on-water cross-section measurement.

The upstream and downstream ends of the PØD are calorimeter regions, these are composed of alternating direction (x and y plane) scintillator bars and lead sheets. The centre of the PØD is then the water target region, this has a similar design, but substitutes the lead for brass and an additional water bag layer, see Figure 2.13. Throughout the PØD triangular scintillator bars, each 33 mm wide by 17 mm high, are used for increased tracking precision. Each bar is read out from a WLS fibre at one end, with the opposing end being mirrored; the scintillator material, WLS fibre and read out is the same as the other ND280 scintillator sub-detectors.

Separating the detector out into the water target and calorimetry regions allows good containment of electromagnetic showers emanating from the water target region, whilst vetoing interactions occurring in other regions of the ND280. A sim-ilar function is also played by the PØD Electromagnetic Calorimeter (PØDECal) which surrounds the PØD; the PØDECal will be discussed in detail shortly.

FGD

The tracker region of the ND280 uses two Fine Grained Detectors as the target mass which are sandwiched between three Time Projection Chambers. The FGDs are each 1.1 t and have external dimensions of 230 cm wide by 240 cm high

Figure 2.13: Cross sectional view of the PØD showing the position of the upstream and downstream calorimeters and intervening water target region along with the position of the triangular shaped bars used through the sub-detector [66].

scintillator sandwiched around water layers. This gives each FGD approximately 0.85X0 of material depth.

Like the PØD, the FGD uses single ended read out from the scintillator bars with the opposing end being mirrored for increased light yield. The bars have a square end, 9.61 mm by 9.61 mm, and are all 1864.3 mm in length; the scintillator material, WLS fibre and read out is the same as the other ND280 scintillator sub-detectors. The bars form layers in the x and y direction, with each layer made up of 192 bars. A pair of layers, one in x and one in y, is called a module and there are 15 modules in FGD1 and 7 in FGD2.

The presence of water in FGD2 is important for making on-water cross-section measurements for a range of neutrino interaction final states. These can then be used in limiting the uncertainty on the predicted event rate at the far detector.

The water is held within target layers that have a thickness of 2.5 cm and are made of rigid, corrugated Sunlite^r polycarbonate panels. The water in the target layers are held at below atmospheric pressure to prevent any water leaks that may occur from seeping into the detector.

The FGD characteristics of continuous read out, the highest precision timing of any sub-detector, completely active detector material, and a high enough gran-ularity to allow highly accurate vertexing and track reconstruction allows very precise final states to be selected. This will allow a vast array of electron- and muon-(anti)neutrino cross-sections to be measured by the ND280 on water and scintillator target materials.

TPC

Surrounding the FGDs are the three Time Projection Chambers of the ND280 tracker region. The TPCs are all identical in construction and are named TPC1, TPC2 and TPC3 moving from the upstream to the downstream ends of the tracker. The TPCs are designed to precisely track charged particles in three dimensions that emerge from neutrino interactions in the FGDs. The 0.2 T

mag-Figure 2.14: Simplified cut-away diagram of a TPC showing the detection volume, cathode components, read out components, neutrino beam direction, and electric and magnetic field direction [68].

netic field curves the path of charged particles as they pass through the TPC chambers, this allows the momenta of particles, along with the energy loss as a function of distance travelled (dE/dx), to be measured. Combining this in-formation with the known particle energy loss rates as a function of momentum allows the TPC to produce particle identification hypotheses as described in Sec-tion 3.3.1.

Each TPC is built of an inner and outer volume, with the outer volume being 2302 mm wide by 2400 mm high and 974 mm deep [68]. The inner volume contains the Argon-based drift gas (Ar:CF4:iC4H10 in the ratio 95:3:2), whilst the outer volume contains CO2 as an insulator. The inner volume is split down the centre (in the zy plane) by the central cathode of the detector, and combined with a strip pattern machined into the copper walls of the inner-volume produces a uniform electric field across the TPC volumes which is precisely aligned with the detector’s magnetic field. See Figure 2.14 for a simplified diagram of a TPC module.

As a charged particle passes through a TPC it ionises the gas, releasing electrons which drift away from the central cathode to the read out planes. The electrons

plane and these are organised into two, slightly offset, vertical columns. Each MM module is 342 mm by 359 mm and is segmented into 1728 anode pads which are 7.0 mm (vertical) by 9.8 mm (horizontal) each.

The horizontal and vertical positions read out from the MM modules, combined with the hit times and known ionisation drift velocity in the modules allows the precise tracking of multiple simultaneous tracks through the TPCs, and allows matching of tracks to objects reconstructed in the other ND280 sub-detectors.

ECal

The Electromagnetic Calorimeters of the ND280 surround the detector on all faces except the upstream end of the detector, and are broken down into three primary sections. These are the Downstream ECal (DsECal), the Barrel ECal (BrECal), which together make up the tracker ECal, and the PØD ECal. The PØDECal resides between the PØD and the magnetic coils, similarly the BrECal resides between the tracker region of the detector (FGDs and TPCs) and the magnetic coils, and the DsECal makes up the downstream face of the basket be-hind TPC3. All the ECal modules are scintillator and lead sampling calorimeters which provide measurement of nearly all particles exiting the basket detectors.

The BrECal and PØDECal each have six constituent modules, whereas the DsE-Cal is just composed of a single module. All the modules use the same scintillator material, WLS fibre and read out as the other ND280 scintillator sub-detectors, but employ rectangular bars with a cross-section 10 mm by 40 mm which vary in length depending upon module and bar orientation [69]. All the ECal mod-ule materials are housed between carbon fibre sheets with aluminium support structures. The edges are then walled with aluminium sheets to which the elec-tronics, power distribution bars, dry-air circulation and water cooling systems are attached.

The PØDECal serves to tag particles exiting the PØD and distinguish between e^±/γ and µ, but is not required to do full track/shower reconstruction as this

depth, with only six lead-scintillator layers in each module, and all of the bars are aligned with the beam direction and read out from one end. To compensate for the low modules depth, the lead used in the detector is the thickest of the three ECal sections at 4 mm per sheet. This gives each module 4.3 X0 of ma-terial, this quantity was decided upon through Monte Carlo simulations which aimed to optimise photon detection efficiency, shower containment and particle discrimination [70]. The modules are arranged so that there are two above the PØD (one in each side of the clam), two below, and one larger module on each side, all of which use bars that are 2454 mm in length.

The BrECal and DsECal are designed to complement the tracking capabilities of the TPCs and FGDs, and as such are large tracking ECal modules capable of full three-dimensional reconstruction of tracks and particle showers. This means neutral particles, including π⁰ decay photons, can be searched for and their energy measured, along with the charged particles from interactions in the tracker region.

The BrECal modules have 31 lead-scintillator layers, corresponding to 9.7 X0, and the DsECal has 34 layers, corresponding to 10.6 X0. The lead in the tracker modules is 1.75 mm thick and this was chosen following studies of π⁰ decay photon detection efficiency. The BrECal modules are laid out around the tracker region in the same manner as the PØDECal modules, but the bars in the BrECal and DsECal modules are orientated in alternating directions, rotated by 90° to one another. The bars in the DsECal are aligned alternately in the x and y directions and are all 2000 mm long and read out from both ends. For the BrECal there are long bars that align with the beam direction (z direction), which are 3840 mm long and read out from both ends, these are then alternated with shorter, single end read out bars in the perpendicular direction. In the top and bottom modules the perpendicular bars align with the x direction and are 1520 mm long, for the side modules these bars align with the y direction and are 2280 mm long.

The DsECal module was the first to be constructed and in 2009 it was shipped to CERN to be placed in the T9 testbeam to check the operation of the integrated module systems and collect a reference data sample. During the testing the module was subjected to a mixed beam of pions, electrons and protons with

particle identification algorithms that are currently used by the experiment. The DsECal module was then transported and installed within the ND280 for use during T2K data taking Run 1. The remaining BrECal and PØDECal modules were installed the following year for use during T2K data taking Run 2.

Electronics

INGRID and the ECals, SMRD and PØD sub-detectors of the ND280 all use the Trip-T electronics read out system [71] in conjunction with MPPCs (as discussed in the next section) to record data. The electronics hierarchy of the Trip-T detectors is such that individual MPPCs are readout by a Trip-T chip, these chips are capable of reading data from up to 16 individual MPPCs simultaneously. Each Trip-T chip is then read out by a Trip-T Frontend Board (TFB), a single TFB is capable of reading out up to 4 Trip-T chips. Each TFB then has its data collated by a Readout Merger Module (RMM), and a single RMM can merge data from up to 48 TFBs. The RMMs pass the data from each event to Frontend Processing Nodes (FPNs), two RMMs per FPN, which work to combine and compress the data from all sub-detectors for each event taken.

The RMMs also work to control when the data is read out from the MPPCs

The RMMs also work to control when the data is read out from the MPPCs